Water, wastewater, and industrial filtration units typically have an underdrain system for supporting filter media and to space the filter media apart from the bottom of the filter. In addition to providing support for the filter media, the underdrain system serves two primary purposes: to collect the filtered water that passes through the media and to uniformly distribute backwash water, backwash air, or a combination of both, across the filter.
Underdrain blocks are often made of pre-cast concrete blocks or concrete filled plastic jacketed blocks. The high-strength concrete blocks are placed side-by-side and end-to-end in the bottom of the filter to form a “false bottom”. The blocks provide support for the filter media and are shaped to collect the filtered water that passes through the media.
In addition to shaping for collection, the blocks have conduits, commonly known as laterals, to allow for the passage of air or water used as a backwash. Piping, such as air headers, is part of the backwash air distribution system. Backwash water is brought into the system by flumes. Air headers bring in pressurized air, which is distributed into the laterals by pipes from the top of the headers, through the flumes and into the laterals.
During backwash operations, air is forced through the air distribution pipes and into the air laterals of the underdrain blocks. The pressurized air can be used as a backwash or combined with water for an air/water backwash. The pressurized air/water combination causes the filtered water to be passed upward through the media with sufficient velocity to prevent filter problems such as mud balls, filter cracking, agglomeration buildup on the media grains, and inactive areas within the filter. The underdrain blocks also act to physically separate the filter media from the air distribution pipes to prevent separate the air distribution pipes to prevent clogging of the orifices in the pipes by the finer particles of the filter media.
Problems can occur because the upward pressure against the blocks can be too forceful. Sometimes the pressure can fall within a range of 2 psi to 6 psi. Such strong pressure tends to dislocate the blocks, thereby allowing the filter media to drop into and damage the infrastructure. The cost of emptying the filter and realigning the blocks is high both in manpower and downtime of the filter.
U.S. Pat. No. 6,325,931 to Roberts discloses one example of an extruded underdrain block that is equipped with side rails. The side rails function to help align the blocks side-to-side in the filter basin. In one embodiment, the side rails on one side are offset from the side rails on the other side to allow the block to interlock with the adjacent block, eliminating the need for grout between the blocks.
U.S. Pat. No. 6,143,188 to Jantsch et al. discloses an underdrain block for a filter media system. The underdrain block is comprised of a top wall, a bottom wall, two side walls and plurality of interior walls. Jantsch also describes filling the gaps between the blocks with grout. The grout is used to hold the blocks together.
U.S. Pat. No. 4,923,606 to Gresh et al. Gresh et al. disclose underdrain blocks that are a combination of a plastic jacket and a concrete mold. The underdrain blocks of the '606 reference are used for both upflow and downflow filtration units. Thus the blocks provide for a distribution of both liquid and air during backwash operations.
U.S. Pat. No. 5,160,614 to Brown discloses an underdrain system comprised of a series of modular, interconnected air duct blocks. In the '614 reference, the row of air duct blocks extend transversely to a plurality of laterals. The laterals are defined by the connection of individual underdrain blocks in parallel adjacent rows. The air duct blocks are interconnected such that interiors of the separate air duct blocks are in fluid communication with one another to supply backwash gas through a single source of gas supply.
Because stability in the support surface is important during backwash, there is a need for an improved underdrain system that will limit the movement of the underdrain blocks during backwash procedures. Existing underdrain block designs offer either no features for backwash stability or only a limited amount of backwash stability through offset rails.